The present invention relates generally to a radio frequency (RF) distribution network and in particular to a modular radio frequency signal tapping system that is useful in a broadband signal transmission network.
Broadband networks are commonly used to transport video signals and digital data to users who may be distributed over a wide area. This application is primarily concerned with a particular type of broadband network, a cable television distribution system. However, the principles set forth herein are generally applicable to other types of broadband network. In a typical cable television distribution system, several television signals, each having a bandwidth of approximately 6 MHz, are frequency division multiplexed onto one coaxial cable by modulating respective radio frequency carrier signals with the television signals. These carrier signals may have frequencies ranging from 5 MHz to 550 MHz in increments of 6 MHz. The combined signal is distributed to the public via a coaxial cable transmission network.
Due to frequency dependent transmission losses in coaxial cables, signal levels tend to diminish as the signal propagates away from the transmission source. The losses at higher frequencies are generally greater than those at lower frequencies. To compensate for these transmission losses, amplifiers are placed between coaxial cable spans in the cable system to restore the signal to its original amplitude. These amplifiers are configured to have a frequency response characteristic that tends to amplify the high frequency components of the signal more than the low frequency components to equalize the transmission losses. A typical cable network may include as many as 30 amplifiers in one line and may have cable spans of up to 1500 feet between the amplifiers.
A tap is inserted in the coaxial cable at locations along a cable span where the transmitted signal is provided to one or more subscribers. As used in this description, a tap is a signal coupler which sends a prescribed amount of signal energy from the cable to a subscriber and passes the remainder to subscribers farther along the network. A typical tap may have outlets for up to eight subscribers. The amount of signal energy provided to a subscriber depends on two factors, the available signal energy at the tap and the coupling coefficient of the tap. As used herein, the coupling coefficient of a tap is the amount of signal energy provided to the subscribers divided by the amount of signal energy applied to the tap. Generally, taps having a relatively small coupling coefficient are used near the start of a cable span, where the signal is strong and taps having a relatively large coupling coefficient are used near the end of the cable span where the signal is weak.
A cable television network is desirably configured so that each subscriber receives approximately the same signal level. Accordingly, a cable network designer wants to have a relatively large number of different types of tap modules available for his use. For example, a typical cable television network may use taps having 12 different coupling coefficients and four different subscriber outlet configurations (i.e. for two, four, six or eight subscribers). Thus an equipment manufacturer may be asked to supply its customers with a total of 48 different tap types.
Moreover, during normal operation, taps in the network may fail and need to be replaced. Since most cable operators rely on customer complaints to determine when the system has failed, it may be relatively difficult to determine exactly which tap in the network has failed. Accordingly, the cable operator may need to stock his service trucks with one tap of each type to ensure that any fault may be corrected with one service call.
When a tap is serviced, it is often removed and replaced. If this operation breaks the signal connection to the subscribers farther down the line, the operator may receive many unnecessary complaints from the affected subscribers.